Field of the Invention
The present invention generally relates to land-mobile voice and data communications, such as in a cellular and personal communication systems, and more particularly to circuitry to control the power level of mobile units.
Description of the Prior Art
Signal amplifiers in general, and RF amplifiers for land-mobile applications in particular, are generally controlled to provide a power output at one of several predetermined signal levels. Such controls are generally required by administrative agency regulations which are expected to become more stringent in the future. A basic and crude way to achieve output power level control is illustrated in FIG. 1, for the example of an RF amplifier 10. The RF input signal is applied to the amplifier 10 via an adjustable attenuator 11 (or a variable gain amplifier). The output of the amplifier is sampled by means of a directional coupler 12 and detected by a Schottky diode 13, the output of which provides a measure of the output level of the amplifier 10. The diode detector 13 is essentially a current rectifier that transforms a small portion of the AC voltage at the output of the amplifier (via the coupler 12) into a DC voltage that is proportional to the output power of the amplifier. This detected voltage is then compared in comparator 14 against a set point voltage representing the desired output power. The output of the comparator 14 then drives the signal attenuator 11 (or variable gain amplifier) to adjust the overall gain of the amplification and thereby the output power level of the amplifier 10. Thus, this negative feedback control system acts as a servo mechanism in such a way that the feedback voltage from the diode detector 13 will equal the set point control voltage so that the output power level of the amplifier 10 is adjusted to a desired level.
The problem with the basic circuit shown in FIG. 1 is that the output power metering element, the Schottky diode detector 13, is temperature dependent such that the detected voltage will vary with the ambient temperature at a rate of about 2 millivolts per degree Celsius. When the amplifier 10 is set to generate a very low output power level and under extreme temperature conditions, the detected voltage will be very small, and a large temperature induced voltage can significantly fool the voltage detection causing a false reading of the output power. The control loop will act on this false reading and produce an incorrect output power level.
To remedy the above problem, another conventional technique uses a second diode whose electrical characteristics precisely match those of the detecting diode. The second diode is normally placed in thermal proximity of the detecting diode so that it will develop a similar thermally induced voltage. The second diode's function is not to rectify (or detect) the RF power from the amplifier but, rather, merely to produce a thermally generated voltage that will be subtracted from the main detector voltage (using a voltage subtracting circuit) leaving only the true power indicating voltage. An example is shown in U.S. Pat. No. 4,523,155 to Walczak et al. which discloses a temperature compensated automatic output control circuitry for power amplifiers which uses matched diode temperature compensation.
This technique works better than the one described above wherein only a single diode is used but still suffers from a number of shortcomings. First, a matched diode pair is expensive, and often the pair must be hand selected from a batch of diodes for close matching of temperature characteristics. Second, since diode detectors are typically biased with a small DC current to improve their sensitivity, each diode in the diode pair is biased with the same amount of DC current and separated by an RF isolation network, with the simplest being an RF choke. Here, a problem arises, especially at higher RF frequencies, that some RF energy will "leak" from the main detecting diode to the compensator diode (i.e., the second diode) causing some AC rectification to take place in it where only DC current is supposed to exist. This RF leakage compromises the effectiveness of the dual diode scheme.